The present invention relates to a metal halide discharge lamp, a lighting device for a metal halide discharge lamp, and an illuminating apparatus using a metal halide discharge lamp.
A metal halide discharge lamp comprises a light-emitting tube provided with a pair of electrodes arranged to face each other. A rare gas, a halide of a light-emitting metal, and mercury are sealed in the light-emitting tube to form the metal halide discharge lamp. The discharge lamp of the particular construction exhibits a relatively high efficiency and high color rendering properties and, thus, is used widely.
The metal halide discharge lamp is classified into a short arc type and a long arc type. The short arc type metal halide discharge lamp is used in projectors such as a liquid crystal projector in which light rays emitted from a lamp are collected so as to be projected onto a screen, and an overhead projector, and is also used for illumination of shops in the form of down light and spot light. Also, a small short arc type metal halide discharge lamp has come to be used in recent years as a headlamp of a vehicle in place of a halogen lamp.
As described in, for example, Japanese Patent Disclosure (Kokai) No. 2-7347, it is absolutely necessary to seal about 2 to 15 mg of mercury in a metal halide discharge lamp used as a headlamp of a vehicle.
On the other hand, Japanese Patent Disclosure No. 3-112045 discloses a metal halide discharge lamp which does not necessitate the sealing of mercury. In this prior art, a rare gas such as helium or neon is sealed in the lamp at a pressure of 100 to 300 Torr in place of mercury so as to obtain a desired lamp voltage. Since the atom of each of these rare gases has a small radius, the rare gas permeates through a quartz glass and, thus, the hermetic vessel of the lamp is formed of a transparent ceramic material.
On the other hand, the long arc type metal halide discharge lamp is used mainly for the general illumination purposes. For example, the discharge lamp of this type is used as an illumination equipment for a high ceiling, a light projector, a street lamp and as an illumination equipment for roads. Further, a metal halide discharge lamp which generates ultraviolet rays is used for the manufacture of a photo-setting synthetic resin or ink. The metal halide discharge lamp used for this purpose is also of a long arc type.
In any of the short arc type and long arc type metal halide discharge lamps which have been put to practical use nowadays, it is absolutely necessary to use mercury, because, in the metal halide discharge lamp, mercury serves to obtain a desired lamp voltage so as to maintain satisfactory electric properties.
To be more specific, where, for example, the lamp voltage is unduly low, the lamp current must be increased in order to obtain a desired lamp input. In this case, problems are brought about that the current capacities are increased in the related facilities such as the lighting device, illuminating device and wiring. Also increased is the heat generation.
On the other hand, where the lamp current is unduly high, the electrode loss is increased, leading to a low lamp efficiency. To be more specific, the electrode drop of the metal halide discharge lamp is constant for each lamp. As a result, if the lamp voltage is unduly low, the lamp current must be increased for making up for the low lamp voltage, with the result that the electrode loss is increased in proportion to the lamp current so as to lower the lamp efficiency.
As pointed out above, it is generally advantageous in a discharge lamp to set the lamp voltage at a value as close to the input voltage of the lamp as possible, i.e., as high as possible, as far as the arc does not disappear.
Let us describe the reason why the mercury sealing was required in the conventional metal halide discharge lamp while giving consideration to the lamp voltage with reference to FIG. 1. As shown in the drawing, the lamp comprises a hermetic vessel 1, a pair of electrodes 2, 2, and lead wires 3, 3. The lamp voltage V1, which denotes the voltage between the lead wires 3, 3 when the metal halide discharge lamp is lit, can be represented by formula (1) given below:V1=E×L+Vd  (1)
where E is a degree of potential inclination of the plasma between the electrodes, L is a distance between the electrodes, and Vd is an electrode drop.
The potential inclination degree E of the plasma can be represented by formula (2) given below:E=I/2π∫σrdr  (2)
where I is a lamp current, σ is an electrical conductivity of the plasma, which is a function of temperature T, and r is a distance of an optional point from the center in the radial direction.
If a substance A is supposed to be present within the discharge space during the lighting of the metal halide discharge lamp, the electrical conductivity σ of the substance A at temperature T is given by formula (3) below:σ=C·NE/(T1/2·(NA·Q))  (3)
where C is a constant, NE is an electron density, NA is a density of the substance A, and Q is a cross section of impingement of the electron against the substance A.
As apparent from formula (1), the lamp voltage V1 is increased with increase in the potential inclination degree E and with increase in the distance L between the electrodes. On the other hand, formula (2) indicates that the potential inclination degree E is increased with decrease in the electrical conductivity σ and with increase in the lamp current I. Further, formula (3) indicates that the electrical conductivity σ is decreased with decrease in the electron density NE and with increase in the density NA of the substance A and in the impinging cross section Q. It follows that, where the distance L between the electrodes and the lamp current I are set constant, the conditions of the substance A, under which the lamp voltage V1 is increased, are that the substance A is unlikely to be ionized to diminish the value NE, that the substance A has a high density within the lamp to increase the value NA, and that substance A has a large cross section Q of the electron impingement.
It should be noted that mercury has a very high vapor pressure, i.e., 1 atmosphere at 361° C., is unlikely to be ionized, and has a large cross section of the electron impingement. It follows that a desired lamp voltage can be easily obtained by controlling the sealing amount of mercury in accordance with the size of the lamp. In other words, mercury is sealed in the conventional metal halide discharge lamp because a desired lamp voltage can be obtained easily.
It should be noted in this connection that, in the case of a metal halide discharge lamp, it is necessary to set the mercury vapor pressure higher with miniaturization of the lamp in which the distance L between the electrodes is shortened in order to ensure a desired lamp voltage. For example, in a small short arc type metal halide discharge lamp whose light-emitting tube has an inner volume of 1 cc or less, the mercury vapor pressure during lighting of the lamp is as high as at least 20 atmospheres.
Let us describe the problems which are brought about where mercury is sealed in a metal halide discharge lamp and the problems which are brought about where mercury is not sealed in the conventional metal halide discharge lamp.
Problems Brought about by Mercury Sealing:
Air pollution and water contamination problems attract worldwide attentions nowadays. Since mercury is harmful to the health of the human being, it is naturally desirable to decrease the amount of mercury used or not to use mercury at all in the field of illumination. In other words, the greatest problem inherent in the conventional metal halide discharge lamp is that mercury is sealed in the lamp.
In addition, many problems remain unsolved when it comes to the metal halide discharge lamp in which mercury is sealed for obtaining a desired lamp voltage, as pointed out below:
1. Poor in Rising of Spectral Characteristics in the Start-up Time:
Where a metal halide discharge lamp is used in the headlamp of a vehicle, required is the instant rising of the light flux. To meet this requirement, employed is a lighting system in which xenon is sealed as a starting gas at a high pressure, and a large current is allowed to flow in the initial period of the lighting, followed by gradually decreasing the current with time. It is certainly possible to achieve the instant rising of light flux in this fashion. However, since mercury is rapidly evaporated in the switch-on time, mercury takes much energy so as to cause delay in the rising of the vapor pressure of a light-emitting metal. It follows that mercury continues to emit light of high intensity for 10 to 20 seconds. It should be noted that the light emitted from mercury is poor in color characteristics and in color rendering properties. Also, the chromaticity of the light emitted from mercury fails to fall within a range of whiteness. Since the rising of the spectral characteristics is very poor as described above, it takes a long time to obtain emission of light having desired spectral characteristics.
2. Unsuitable for Light Control (Dimming):
A change in temperature of the light emitting tube brings about a great change in the color temperature of the emitted light and, thus, in the color rendering properties, as apparent from FIG. 2. Specifically, FIG. 2 is a graph showing the distribution of an emission spectral of a conventional short arc type metal halide discharge lamp for projection. The wavelength (nm) is plotted on the abscissa of the graph, with the relative emission power (%) being plotted on the ordinate.
Sealed in the conventional short arc type metal halide discharge lamp are 6.65×104 Pa of argon as a rare gas, 1 mg of dysprosium iodide (DyI3) as a halide, 1 mg of neodymium iodide (NdI3) as a halide, and 13 mg of mercury. The emission spectral consists of a continuous light emission caused by dysprosium and neodymium and main bright-line spectra caused by the elements given above the arrows in the drawing. As seen from the graph, the bright-line spectrum caused by mercury has a large power.
It should be noted that the amount of light emission from each of the light-emitting metals is changed proportionally to the vapor pressure within the lamp. Since the vapor pressure of a halide of a light-emitting metal is markedly lower than that of mercury, a change in temperature of the light emitting tube causes a change in the evaporation amount of the halide, leading to a change in the vapor pressure within the lamp. As a result, the amount of light emitted from the light-emitting metal is also changed.
On the other hand, the vapor pressure of mercury is so high that a change in temperature of the light emitting tube does not bring about an appreciable change in the mercury vapor pressure, leading to a small change in the amount of light caused by the strong bright-line spectrum of mercury. It follows that, if the input power supplied to the light-emitting tube is decreased, the light emission caused by mercury is rendered relatively predominant. As a result, the color temperature of the emitted light is lowered, leading to poor color rendering properties. This indicates that the conventional metal halide discharge lamp, which requires mercury sealing, is unsuitable for the light control (dimming).
In the case of a headlamp for a vehicle, dimming is required for the lighting in the day time (day light) employed in the U.S.A. and Europe. However, the color characteristics are markedly impaired in the conventional metal halide discharge lamp requiring the mercury sealing.
3. Large Unevenness in Properties:
The metal halide discharge lamps having mercury sealed therein are uneven in temperature of the light emitting tubes, which are caused by unevenness in the size of the individual lamps. As a result, unevenness in the characteristics tends to be brought about even under the same input power. Also, the characteristics are likely to be changed by the temperature elevation in the coolest region caused by the blackening of the light-emitting tube used over a long period of time. These difficulties tend to bring about a problem particularly where a plurality of metal halide discharge lamps are used in combination for illumination as in, for example, shops.
4. Difficult to Re-Start up Instantly:
As described previously, the distance between the paired electrodes is small in a short arc type small metal halide discharge lamp, making it necessary to set the mercury vapor pressure at a high value. Specifically, the mercury vapor pressure is set at such a high value as, for example, at least 20 atmospheres.
Further, when used in a headlamp for a vehicle, xenon is also sealed in the lamp at a high pressure. For example, the xenon pressure is as high as about 35 atmospheres during the lighting. Since the mercury vapor pressure and the xenon vapor pressure are very high during the lighting, it is necessary to apply a pulse voltage of a very large power in the re-start up time. It follows that the lighting circuit is rendered costly. In addition, it is necessary to insulate the circuit, the lamp and the equipment housing them against a high voltage.
5. Rupture of Light-Emitting Tube
Since the mercury vapor pressure is very high during the lighting as described previously, strain of the lamp is increased during lighting of the lamp over a long period of time, with the result that the lamp tends to be ruptured. The problem of rupture markedly lowers the reliability of the lamp.
6. Low Screen Brightness when Used in Projector:
Where a short arc type metal halide discharge lamp is used in a projector in which the light emitted from the lamp used as a light source is collected through an optical system such as a liquid crystal projector so as to illuminate, for example, a screen arranged apart from the projector, it is of high importance to suppress the loss of the light emitted from the discharge lamp when the emitted light passes through the optical system so as to permit the emitted light to arrive at the screen as much as possible. In order to improve the brightness of the screen by suppressing the light loss, it is necessary for the arc of the discharge lamp to be narrowed thin. The expression “narrow arc” denotes that the arc temperature distribution is sharp.
It should be noted in this connection that the light emitted from mercury is absorbed and, thus, is optically thick. Since energy is absorbed by the absorption of the light emitted in the intermediate and low temperature regions, the temperature is elevated. As a result, the arc temperature is distributed to depict a parabola, making it difficult to narrow the arc. On the other hand, it is known to the art that, if the light emission is very much increased by using scandium or a rare earth metal as a light-emitting metal, the arc can be narrowed even in the presence of mercury. In this case, however, convection occurs vigorously if the lighting pressure of mercury is high, with the result that the arc is rendered unstable. It follows that it is impossible to put this technique to practical use.
Problems Inherent in the Conventional Lamp which does not Necessitate Mercury Sealing:
In the metal halide discharge lamp which does not necessitate mercury sealing, the partial pressure of helium or neon within the light-emitting tube is markedly increased during the lighting. If the light-emitting tube is constructed to withstand the high pressure, it is certainly possible to obtain a metal halide discharge lamp in which mercury is not sealed. The possibility itself of obtaining a metal halide discharge lamp not requiring mercury sealing is worthy of a favorable evaluation. However, it is practically difficult to permit a metal halide discharge lamp of the construction similar to that of the conventional lamp to withstand the high pressure within the lamp during the lighting of the lamp. Where, for example, a lamp voltage of 50 to 60V is required in s small metal halide discharge lamp, the pressure of helium or neon within the lamp is expected to exceed 150 atmospheres during the lighting of the lamp. It follows that the hermetic vessel widely used in the conventional lamp fails to obtain a high reliability in respect of the measure against rupture of the hermetic vessel.